Decision Structures CSC1310. Python Programming, 2/e 1

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Theodore Oliver
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1 Decision Structures CSC1310 Python Programming, 2/e 1

2 Simple Decisions Decision structures, which are statements that allow a program to execute different sequences of instructions for different cases, allowing the program to choose an appropriate course of action. Python Programming, 2/e 2

3 Example: Temperature Warnings Let s return to our Celsius to Fahrenheit temperature conversion program. Let s say we want to modify that program to print a warning when the weather is extreme. Any temperature over 90 degrees Fahrenheit: Hot weather warning Lower than 30 degrees Fahrenheit: Cold weather warning Python Programming, 2/e 3

4 Example: Temperature Warnings The Python if statement is used to implement the decision. if <condition>: <body> The body is a sequence of one or more statements indented under the if heading. Python Programming, 2/e 4

5 Example: Temperature Warnings The body of the if either executes or not depending on the condition. In any case, control then passes to the next statement after the if. This is a one-way or simple decision. Python Programming, 2/e 5

6 Example: Temperature Warnings The body of the if either executes or not depending on the condition???. In any case, control then passes to the next statement after the if. This is a one-way or simple decision. Python Programming, 2/e 6

7 Forming Simple Conditions What does a condition look like? At this point, let s use simple comparisons. <expr> <relop> <expr> <relop> is short for relational operator Python Programming, 2/e 7

8 Forming Simple Conditions Python Mathematics Meaning < < Less than <= Less than or equal to == = Equal to >= Greater than or equal to > > Greater than!= Not equal to Python Programming, 2/e 8

9 Forming Simple Conditions Notice the use of == for equality. Since Python uses = to indicate assignment, a different symbol is required for the concept of equality. A common mistake is using = in conditions! Python Programming, 2/e 9

10 Forming Simple Conditions Conditions are based on Boolean expressions. When a Boolean expression is evaluated, it produces either a value of true (meaning the condition holds), or it produces false (it does not hold). Some computer languages use 1 and 0 to represent true and false. Python Programming, 2/e 10

11 Two-Way Decisions In Python, a two-way decision can be implemented by attaching an else clause onto an if clause. This is called an if-else statement: if <condition>: <statements> else: <statements> Python Programming, 2/e 11

12 Two-Way Decisions When Python first encounters this structure, it first evaluates the condition. If the condition is true, the statements under the if are executed. If the condition is false, the statements under the else are executed. In either case, the statements following the ifelse are executed after either set of statements are executed. Python Programming, 2/e 12

13 Multi-Way Decisions Imagine if we needed to make a five-way decision using nesting. The if-else statements would be nested four levels deep! There is a construct in Python that achieves this, combining an else followed immediately by an if into a single elif. Python Programming, 2/e 13

14 Multi-Way Decisions if <condition1>: <case1 statements> elif <condition2>: <case2 statements> elif <condition3>: <case3 statements> else: <default statements> Python Programming, 2/e 14

15 Multi-Way Decisions This form sets of any number of mutually exclusive code blocks. Python evaluates each condition in turn looking for the first one that is true. If a true condition is found, the statements indented under that condition are executed, and control passes to the next statement after the entire if-elifelse. If none are true, the statements under else are performed. Python Programming, 2/e 15

16 Multi-Way Decisions The else is optional. If there is no else, it s possible no indented block would be executed. Python Programming, 2/e 16

17 Study in Design: Max of Three Suppose we need an algorithm to find the largest of three numbers. Python Programming, 2/e 17

18 Strategy 1: Compare Each to All This looks like a three-way decision, where we need to execute one of the following: max = x1 max = x2 max = x3 All we need to do now is preface each one of these with the right condition! Python Programming, 2/e 18

19 Strategy 2: Decision Tree We can avoid the redundant tests of the previous algorithm using a decision tree approach. Python Programming, 2/e 19

20 Strategy 3: Sequential Processing How would you solve the problem? You could probably look at three numbers and just know which is the largest. But what if you were given a list of a hundred numbers? One strategy is to scan through the list looking for a big number. When one is found, mark it, and continue looking. If you find a larger value, mark it, erase the previous mark, and continue looking. Python Programming, 2/e 20

21 Strategy 4: Use Python Python has a built-in function called max that returns the largest of its parameters. Python Programming, 2/e 21

22 Some Lessons Your first task is to find a correct algorithm. Think about the design and ask if there s a better way to approach the problem. Strive for clarity, simplicity, efficiency, scalability, and elegance. Python Programming, 2/e 22

23 Some Lessons Be the computer. One of the best ways to formulate an algorithm is to ask yourself how you would solve the problem. This straightforward approach is often simple, clear, and efficient enough. Python Programming, 2/e 23

24 Some Lessons Generality is good. Consideration of a more general problem can lead to a better solution for a special case. If the max of n program is just as easy to write as the max of three, write the more general program because it s more likely to be useful in other situations. Python Programming, 2/e 24

25 Some Lessons Don t reinvent the wheel. If the problem you re trying to solve is one that lots of other people have encountered, find out if there s already a solution for it! As you learn to program, designing programs from scratch is a great experience! Truly expert programmers know when to borrow. Python Programming, 2/e 25

Multi-Way Decisions. The newest program is great, but it still has some quirks! This program finds the real solutions to a quadratic

Multi-Way Decisions. The newest program is great, but it still has some quirks! This program finds the real solutions to a quadratic Multi-Way Decisions The newest program is great, but it still has some quirks! This program finds the real solutions to a quadratic Please enter the coefficients (a, b, c): 1,2,1 The solutions are: -1.0-1.0